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Magnetic stimulator systems have a wide range of medical applications, including transdermal nerve stimulation. In many cases a monophasic stimulus is desired, in which each magnetic pulse in a pulse train has a rapid rise and a slow fall. In contrast, a pulse having similar rise and decay rates is referred to as being biphasic.
The general prior art approach for generating such pulses is illustrated in FIG. 1. A stimulus coil 12 acts as an inductance in the circuit 10, and the magnetic field at the coil 12, which is proportional to the coil current, is the output stimulus. A pre-charging power supply 14 charges a capacitor C to a predetermined voltage. A switch S1 is used to connect the coil 12 across the capacitor C, causing an LC oscillation with a fast current rise in the coil 12. When the capacitor 16 voltage reaches zero, at which point the coil 12 current will be near its peak, a second switch S2 is closed bringing a resistor R into the circuit, causing an LCR resonant decay with a slow fall time.
The magnitude of the current pulse in the coil 12, and thus the resulting magnetic field, is determined by the capacitor C pre-charge voltage. The relative rate of rise and fall of the current, and the shape of the pulse, is determined by the values of the fixed circuit elements, such as the coil 12 inductance, the capacitance of capacitor C, and the resistance of resistor R. All of the capacitor C pre-charge energy is dissipated in the resistor R, coil 12, and switches S1, S2.
One embodiment 20 of this prior art approach, used in the MagPro magnetic stimulator by Medtronic, Inc., Minneapolis, Minn. (formerly Dantec Medical A/S), is illustrated in FIG. 2. Values for the circuit components are: C=180 xcexcf; Lcoil≈11 xcexcH; and R≈60-90 mxcexa9. This system uses a thyristor 24 as the first switch, and a diode 22 as the second switch, providing simple, static control over the coil 28 pulse shape out of the pre-charge power supply 26. Coil current and voltage waveforms with a 220 Vac input pre-charge power supply 26 are shown in FIG. 7.
The prior art approach to generating the desired waveform, while being simple and straightforward, has two serious deficiencies. First, the capacitor pre-charge energy for each pulse is entirely dissipated in the circuit elements, thus requiring a pre-charging power supply that draws a large amount of power from the utility or other source. For many practical applications, the amount of power required for desired pulse magnitudes and repetition rates is greater than can be drawn from a conventional 15 A, 110 V outlet, thus necessitating either a higher-voltage or higher-current utility outlet. Furthermore, the dissipated power results in a large amount of heat loss into the environment which is undesirable and potentially unsafe.
A second deficiency associated with the prior art approach is that the pulse shape and duration at the coil is determined entirely by the values of the constituent circuit elements, values which cannot be adjusted dynamically. In order to enable flexible or adaptive control over the resulting waveform, either monophasic or biphasic, accurate dynamic adjustment of stimulator circuit characteristics such as the relative rise and fall rates of the circuit current must be enabled.
A new stimulator circuit 100 that overcomes the limitations of the prior art is disclosed. The general structure of the present invention is illustrated in FIG. 3. The system 100 has a pre-charge power supply 102, a capacitor C, a set of switches S1, S2, S3, and S4, and a stimulator coil 104. The switches, which can be implemented using a variety of devices as discussed in detail below, enable flexible control over the coil current waveform without requiring the physical reconfiguration of circuit elements. The switches and coil may be collectively referred to as a coil switching circuit 106.
Certain common reference designators are used in multiple drawings, such as the legends xe2x80x9cCxe2x80x9d and xe2x80x9cS1,xe2x80x9d though this is merely for convenience and is not to imply that the devices so designated are necessarily the same in each illustrated embodiment.
In the presently disclosed invention, the shape of the output current pulse is controlled by the modulation of the switches S1 through S4. This contrasts with the prior art, in which the current pulse shape is determined only by the values associated with constituent circuit elements and in which current rise time is not dynamically controllable. Furthermore, with the presently disclosed circuits and methods, much of the energy (limited by parasitic losses in the coil, switching devices, etc.) is returned from the coil to the capacitor for reuse on the succeeding pulse. The presently disclosed invention thus has lower power requirements and produces less heat as compared to the prior art.